Relating to a line on Earth's surface where magnetic declination is zero, meaning a compass needle points to true north rather than magnetic north.
From Greek 'a-' (without) plus 'gonia' (angle), literally meaning 'without angle.' In geography and magnetism, it describes lines where the angle between true north and magnetic north disappears.
There's an invisible line running through the Earth where your compass tells the truth—the agonic line! It shifts over time because Earth's magnetic field is always moving, so maps have to be constantly updated.
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